How to patch Bash on OSX in wake of "shellshock"?

Question

My Mac is vulnerable, as this test shows:

x='() { :;}; echo VULNERABLE' bash -c : (source)

I am using the version of bash that came with it. I also use Homebrew. What is the preferred way to patch Bash?

Answer 1

First, you don't need to do this unless you are are offering web services to the public internet from your Mac. If you are not, wait until there is an official security update from Apple.

However, if you are offering web services, you might want to update.

Official Patch

Apple has released an Official Bash Security Update Here

Checking whether you are vulnerable

To confirm that you are using an outdated bash: $ which bash /bin/bash $ /bin/bash --version GNU bash, version 3.2.51(1)-release (x86_64-apple-darwin13) Copyright (C) 2007 Free Software Foundation, Inc.

The most current bash is 4.3.25

Unofficial Update Method: Compile Bash from Source Code using Homebrew

If you don't have Xcode installed, you'll need the Xcode command line tools, which can be installed by $ xcode-select --install

Or from the developer portal https://developer.apple.com/downloads/index.action?=command%20line%20tools download command_line_tools_for_osx_10.9_september_2014.dmg

To install Brew (http://brew.sh): $ ruby -e "$(curl -fsSL https://raw.githubusercontent.com/Homebrew/install/master/install)"

Then do: $ brew doctor Follow any instructions if there are problems. Many common problems are addressed at http://www.moncefbelyamani.com/how-to-install-xcode-homebrew-git-rvm-ruby-on-mac/#troubleshoot-homebrew

Then update brew to the latest list of packages: $ brew update

To get the latest bash 4.3.25: $ brew install bash

This installs bash into /usr/local/Cellar/bash/4.3.25/bin/bash

The old bash and sh still exists at /bin, so after installing you'll rename the old executables to a new file. $ sudo mv /bin/bash /bin/bash_old $ sudo mv /bin/sh /bin/sh_old

If you are very paranoid, you can remove execute permissions on the bash_old $ sudo chmod a-x /bin/bash_old /bin/sh_old

Then create a symbolic link to the new bash 4.3.25 that brew installed. $ sudo ln -s /usr/local/Cellar/bash/4.3.25/bin/bash /bin/bash $ sudo ln -s /usr/local/Cellar/bash/4.3.25/bin/bash /bin/sh

Reboot and it is complete.

A warning — this may break some existing shell scripts that might rely on bash 3.2 or the differences that the Mac sh has over the linux sh. There is a much more sophisticated answer to replacing bash and sh from sources at https://apple.stackexchange.com/questions/146849/how-do-i-recompile-bash-to-avoid-the-remote-exploit-cve-2014-6271-and-cve-2014-7

In most cases it is best to wait for official updates.

-- Christopher Allen

Answer 2

Please check the version of bash in brew before you do below step:

$ sudo ln -s /usr/local/Cellar/bash/4.3.25/bin/bash /bin/bash

$ sudo ln -s /usr/local/Cellar/bash/4.3.25/bin/bash /bin/sh

The bash version is updated to 4.3.27 so far. if you still link 4.3.25 to /bin/bash, you Mac should not boot. if you have do that, you can boot to recovery mode and copy /bin/bash_old to /bin/bash and copy /bin/sh_old to /bin/sh.

My bash version in brew is 4.3.27, so this will work for me:

$ sudo ln -s /usr/local/Cellar/bash/4.3.27/bin/bash /bin/bash

$ sudo ln -s /usr/local/Cellar/bash/4.3.27/bin/bash /bin/sh

Answer 3

Not good, still vulnerable!

bash-4.3$ brew upgrade bash Error: bash-4.3.25 already installed

bash-4.3$ /usr/local/Cellar/bash/4.3.25/bin/bash bash-4.3$ env x='() { :;}; echo vulnerable' bash -c "echo this is a test"

vulnerable

this is a test